FOLLICULAR DEVELOPMENT IN DOMESTIC RUMINANTS

Abstract

Knowledge on the physiological processes that control follicular development may allow for the development of strategies to increase reproductive efficiency in domestic ruminants. Follicle development depends on the balance between survival factors, proliferation and cell death, which determine whether the follicle starts and continues to grow or is removed from the ovary. During fetal development of the female, primordial germinal cells proliferate by mitosis to reach the gonadal ridge, where the oogonia are surrounded by flattened cells to assemble the primordial follicles. Kit ligand, BMP-15 and GDF-9 stimulates a group of primordial follicles to begin their growth until they reach preantral development. The growth to the antral stage seems to be independent of gonadotrophins and promoted by growth factors. In gonadotrophin suppressed animals follicles will grow up to a diameter of 2 mm in sheep and 4mm in cattle. The cyclic secretion of FSH and LH promotes the recruitment and growth to the large antral stage and the eventual ovulation if luteolysis occurs. Follicular development in cows, sheep and goats during the estrous cycle occurs in a pattern like-wave, where groups of follicles begin their growth in response to an increase of FSH, but only some (sheep and goats) or one (cows) is selected as the dominant, and ovulates if its dominance coincides with the lysis of the CL and the reduction of progesterone. Among the factors that determine whether a large antral follicle starts, continues and completes its development are its responsiveness to gonadotropins, its steoidogenic capability and the presence of survival and proliferating factors such as IGF-I and VEGF.